Radiative properties of up-conversion coatings formed on the basis of erbium-doped barium titanate xerogels

The Stokes and anti-Stokes luminescence of erbium in sol–gel-derived barium-titanate xerogels (BaTiO$_3$ : Er) and BaTiO$_{3}$/SiO$_{2}$ multilayer structures based on these xerogels formed on silicon substrates is studied. It is shown that the up-conversion of erbium in barium-titanate xerogels obtained from erbium nitrate or acetate is observed at an erbium concentration of $>3$ at.%. The up-conversion of erbium is observed at an excitation wavelength of 980 nm and characterized by an intense band in the 520–560 nm range, corresponding to the $^{2}H_{11/2}\to{}^4I_{15/2}$ and $^{4}S_{3/2}\to{}^4I_{15/2}$ radiative transitions of the erbium ion, as well as the bands in the regions of 650 and 850 nm associated with the $^{4}F_{9/2}\to{}^4I_{15/2}$ и $^{4}I_{9/2}\to{}^4I_{15/2}$ transitions, respectively. Based on the obtained BaTiO$_{3}$ xerogels, multilayer BaTiO$_{3}$ : Er/SiO$_{2}$/Si structures are formed on silicon substrates, providing a significant, compared to single-layer BaTiO$_3$ : Er/Si structures, increase in the intensity of erbium luminescence at the wavelength of the main radiative transition (1540 nm). The prospects of developing up-conversion coatings based on erbium-doped barium titanate for practical applications are discussed.